1. Field of the Invention
The present invention particularly relates to an air-fuel ratio control apparatus for engine in which oxygen concentration sensors (O.sub.2 sensors) are respectively disposed upstream and downstream streams of a catalyzer and an air-fuel ratio is controlled in accordance with the output signals of those sensors.
2. Description of the Related Art
As for gasoline engines, there is conventionally known an apparatus in which an air-fuel ratio is controlled to the vicinity of a theoretical air-fuel ratio (or a catalyzer window) in accordance with the output signal of an O.sub.2 sensor disposed upstream of a catalyzer, thereby improving the rate of purification by the catalyzer.
Further, there is an air-fuel ratio control apparatus for gasoline engine in which a change in characteristic of the output signal of an O.sub.2 sensor provided upstream of a catalyzer, or the like, is corrected in accordance with the output signal of an O.sub.2 sensor provided in the lower stream of the catalyzer (for example, see JP-A-61-286550).
On the other hand, the present inventors have conducted experiments on engines using various gases such as a city gas and have revealed that an exhaust gas is not sufficiently mixed even downstream of a catalyzer. This is caused by the fact that the gas is harder to mix in the air, as compared with the gasoline. The above phenomenon remarkably appears, especially, in an apparatus in which an air-fuel ratio is controlled by adjusting an intake air or a fuel gas which is supplied to the upper stream of a throttle valve, by-passing a mixer for mixing the intake air and the fuel gas.
Accordingly, in the case where such air-fuel ratio control as mentioned above is applied to a gas engine, there is a problem that the output of an O.sub.2 sensor disposed upstream or downstream of a catalyzer changes depending upon the attachment position of the O.sub.2 sensor, for example, variations of the attachment position thereof in a direction of circumference of an exhaust pipe, thereby giving rise to variations of the control performance.